Fans are commonly classified as either axial or centrifugal, depending upon the direction in which air is discharged by the fan blades relative to the rotational axis of the impeller. In axial flow fans the air is discharged in a direction substantially parallel to the axis of the impeller, whereas in centrifugal fans, the air is discharged in a substantially radial direction. The present invention relates to an improved impeller and blade design for centrifugal fans.
Centrifugal fans are subclassified in the literature according to their impeller and blade designs. The impeller and blade designs employed in the commercially available centrifugal fans are the airfoil, backward inclined, radial, modified radial, and forward curved. Of these, the airfoil type has been recognized as being the most efficient. The airfoil type centrifugal fan incorporates a plurality of impeller blades which, in cross section, have a shape like an airfoil. The blades are circumferentially spaced around the impeller and are backward inclined. As used herein, the term "backward inclined" means that each blade slopes backwards from the direction of rotation in such manner that the heel of each blade is radially inward of its tip and is forward of the tip relative to the direction of rotation. In centrifugal fans employing airfoil blades, the common practice until now has been to construct and mount the blades so that the convex, or high camber, side of the airfoil faces radially outward. However, two problems have been observed with the airfoil fans constructed in this manner. First, because the blades are backward inclined relative to the direction of rotation with the high camber side of the airfoil facing radially outward, a negative angle of attack is created between the leading edge of the airfoil and the impinging air, thereby reducing the aerodynamic efficiency of the impeller. Second, the natural aerodynamic forces cause the airstream across the inward facing or lower pressure side of the blade to separate from the blade prior to reaching the trailing edge, thereby causing turbulence and undesirable fan noise.
The impellers of the backward inclined centrifugal fans are similarly designed, except that the blades are made of a single thickness of metal, and both the outward and inward facing blade surfaces have substantially the same curvature. The single thickness, backward inclined blades are usually flat, but may be curved, in which case the convex or high camber side of the blades also faces radially outward. These backward inclined centrifugal fans suffer from the same disadvantages as the airfoil fans previously discussed.
In the radial and modified radial fans, the heel and tip of each blade are radially aligned, and the blades comprise a single metal thickness. In the radial fans the blades are planar; in the modified radial fans, each blade is cambered radially inward between the heel and tip, and thus, away from the direction of rotation. Aerodynamically, the radial and modified radial fans are less efficient than either the airfoil or backward inclined fans.
With forward curved fans, the impeller blades typically have a constant thickness and are concave relative to the direction of rotation. However, unlike either the backward inclined or modified radial fans, the tip of each blade extends forward of its heel. Forward curved fans are also less efficient than the airfoil fans.